1. Field of the Invention
The invention relates to a motor driven power steering system in which an auxiliary steering force is generated by an electric motor.
2. Description of the Prior Art
A conventional motor driven power steering system is shown in FIG. 3.
Referring now to FIG. 3, there is shown the general construction of a motor driven power steering system which is of the recirculating ball type. A ballnut 41 is axially slidably fitted in a steering box or casing 40. A drive shaft having a worm gear 42 is engaged with the ballnut 41 via a plurality of balls 43. Therefore, if the worm gear 42 is rotated by the drive shaft, the balls 43 are moved whereby the ballnut 41 is moved in an axial direction.
A rack 41a is formed on an outer circumferential surface of ballnut 41 and is also movable in an axial direction. The rack 41 is engaged with a sector gear 44a which is formed on a sector shaft 44. Accordingly, when the ballnut 41 moves in the axial direction as discussed above, the sector shaft 44 is rotated and the wheels of the vehicle (not shown) are steered via a steering member (not shown).
The worm gear shaft 42 is coaxially arranged with a steering shaft 45 fixed to a steering wheel (not shown). The worm shaft 42 and steering shaft 45 are connected to each other via a torsion bar 46. One end of worm shaft 42 is connected to an electrical motor 48 via a reduction gearing 47. A torque sensor 49 is provided on steering shaft 45. An output signal s.sub.1 in accordance with a rotating direction and rotating torque of steering shaft 45 is supplied to a control apparatus 52 together with an output signal s.sub.2 of speed sensor 14 for detecting the motor vehicle speed. The motor 48 rotates in accordance with a driving signal S.sub.3 supplied from control apparatus 52.
As aforementioned, the auxiliary steering force is produced by the electric motor. However, there is a danger that the motor may burn out due to long periods of use and the rotating unit of the motor could become locked-up due to maintenance problems. In such occurrence, the manual operation of steering is impossible because generally the motor is connected to the steering shaft and the load of the locked motor is applied to the steering shaft.